Conceptual Physics. Lana Sheridan. July 3, De Anza College

Transcription

1 Conceptual Physics Lana Sheridan De Anza College July 3, 2017

2 Overview of the Course Topics What is science? What is physics? How is it done? What is its history?

3 Overview of the Course Topics What is science? What is physics? How is it done? What is its history? What is it useful for? How does it a ect my life? What do scientists do? How do they think?

4 Overview of the Course Topics What is science? What is physics? How is it done? What is its history? What is it useful for? How does it a ect my life? What do scientists do? How do they think? Rational thinking / understanding science articles. Interpreting graphs and data. Basic physics ideas and theories. Applying knowledge to physics problems.

5 Overview of the Course Physics Theory Topics Mechanics and kinematics; Newton s laws; momentum Properties of matter; phases of matter Heat Electricity and Magnetism; circuits and current Light; waves; color

6 Overview of the Course Should I take this course? You should if: You enjoy physics and other technical subjects. You are curious about how nature works. You are willing to use some math. You will be able to attend all or almost all class sessions.

7 Overview of the Course Book Conceptual Physics, 11th Edition, Hewitt You may want another book as well. Assignments Presentation (end of the course). 2Essays. Some collected homework worksheets. Uncollected homework problems from the textbook. (You still need to do them.) Read the textbook. Other reading.

8 Overview of the Course Evaluation Quizzes & Incidental HW 25% Class participation 10% Essay questions 10% Midterm 20% Presentation 10% Projected Grading Scheme Final 25% 88%! 100% = A 75%! 87% = B 60%! 74% = C 50%! 59% = D 0%! 49% = F

9 Overview of the Course O cial Book Conceptual Physics, 11th Edition, Hewitt Other suggested books James S. Walker, Physics Holt, Physics, any edition. Giancoli, Physics: Principles with Applications, any edition.

10 Overview of the Course Note about presentation of calculations Underline, box, highlight, or unambiguously emphasize the answer. For each problem make sure your method is clear. If there is an equation or principle you are using, write it out at the start of your solution. If the reasoning is not clear, the answer is not correct.

11 Science Science is a process for reasoning about the natural world and making predictions for its behavior.

12 Science Science is a process for reasoning about the natural world and making predictions for its behavior. Physicists (and others who use physics) want to predict accurately how an object or collection of objects will behave when interacting. What have people used it for? Why have they cared about it? to better understand the universe to build new kinds of technology (engines, electronics, imaging devices, mass manufacturing, energy sources) to build safer infrastructure to go new places and explore to prepare for the future

13 Early Science: Seasonal Tracking Many early civilizations wanted to accurately track seasons, to aid farming. Newgrange, a Neolithic civilization (3,300 2,900 BC), Ireland. arial view, reconstructed façade Sunlight shaft 1 Left Right from Wikipedia

14 Early Science: Seasonal Tracking Abu Simbal Temple of Rameses II, near Aswan, Egypt The interior statues (except Ptah, an underworld god) are lit on October 21 and February 21, Rameses birthday and coronation day. 1 Right photo from cropped.

15 Early Science: Seasonal Tracking Fajada Butte sun dagger, Chaco civilization (AD ), New Mexico Summer solstice Winter solstice 1 Photo by Harrison Lapahie Jr.

16 Early Science: Seasonal Tracking Uxmal, Maya civilization ( AD), Mexico. Uxmal Governor s palace 1 Left by Palimp Sesto; Rigth by Régis Lachaume.

17 Early Science: Seasonal Tracking The Antikythera mechanism, an ancient Greek calculator for astronomical events ( BC). 1 Left from Wikipedia; right pbs.org.

18 Scientific Statements A scientific fact or scientific statement must be quantitative and falsifiable. quantitiative able to be measured, precise falisifiable able to be proven wrong

19 The Scientific Method The process: 1 Ask a question. 2 Make a guess about the answer: a hypothesis 3 Make predictions based on the guess 4 Do experiments to confirm or disprove the guess IF the guess is wrong: go back to step 2. 5 If the guess is right, formulate it into the simplest possible rule.

20 The Scientific Hypotheses This is not a scientific hypothesis: The outcomes of any experiment are the result of the undetectable influence of a noodly appendage.

21 The Scientific Hypotheses This is not a scientific hypothesis: The outcomes of any experiment are the result of the undetectable influence of a noodly appendage. This is a scientific hypothesis: Near the surface of the Earth, if two objects are dropped from the same height at the same time in a vacuum they will strike the ground at the same time, regardless of their masses.

22 Quantities, Units, Measurement If we want to make quantitative statements we need to agree on measurements: standard reference units. We will mostly use SI (Système International) units: Length Mass Time meter, m kilogram, kg second, s and many more!

23 An Early Scientific Measurement: the Size of the Earth Eratosthenes was a Greek-North African poet, mathematician, geographer and astronomer. He was the chief librarian of the Library of Alexandria. Around 235 BCE he came up with a way to measure the size of the Earth.

24 An Early Scientific Measurement: the Size of the Earth Eratosthenes was interested in geography and created a map of the known world, thought to look like this: 1 Map from Wikipedia.

25 Measuring the Size of the Earth Eratosthenes knew At noon on the summer solstice (June 22) the sunlight shines directly downward in Syene (now called Aswan). Alexandria is 5000 stadia (800 km) north of Syene. The sun casts shadows at that time in Alexandria.

26 Measuring the Size of the Earth 1 Map from scipop

27 Measuring the Size of the Earth 1 Map from Wikipedia.

28 Measuring the Size of the Earth Eratosthenes was able to estimate the size of the Earth: The angle between Syene and Alexandria is 1/50-th of a circle (7.1 /360 ).

29 Measuring the Size of the Earth Eratosthenes was able to estimate the size of the Earth: The angle between Syene and Alexandria is 1/50-th of a circle (7.1 /360 ). That means that the distance from Syene to Alexandria is 1/50-th of the circumference of the Earth! The circumference of the Earth is 40, 000 km. Using C = 2 r, we can find the radius of the Earth, 6370 km.

30 Measuring the Size of the Earth Eratosthenes was able to estimate the size of the Earth: The angle between Syene and Alexandria is 1/50-th of a circle (7.1 /360 ). That means that the distance from Syene to Alexandria is 1/50-th of the circumference of the Earth! The circumference of the Earth is 40, 000 km. Using C = 2 r, we can find the radius of the Earth, 6370 km. Interestingly, Christopher Columbus did not believe this calculation was right...

31 Scientific Statements The size of the Earth is scientific fact, given with some precision. Currently, NASA quotes the polar radius of the Earth as ± 0.05 km equatorial radius of the Earth as ± 0.05 km

32 Some Questions from Hewitt Page 16, # 8. The shadow cast by a vertical pillar in Alexandria at noon on the summer solstice is found to be 1/8 the height of the pillar. The distance from Alexandria to Syene is 1/8 of the Earth s radius. Is there a geometric connection between these two 1-to-8 ratios?

33 Some Questions from Hewitt Page 16, # 8. The shadow cast by a vertical pillar in Alexandria at noon on the summer solstice is found to be 1/8 the height of the pillar. The distance from Alexandria to Syene is 1/8 of the Earth s radius. Is there a geometric connection between these two 1-to-8 ratios? # 9. If the Earth were smaller than it is, but the Alexandria-to-Syene distance were the same, would the shadow of the vertical pillar in Alexandria be longer or shorter at noon during the summer solstice?

34 Other Early Scientific Measurements Aristarchus of Samos was a Greek astronomer and mathematician. He came up with a way to measure 1 the size of the Moon the Moon s distance from the Earth the distance to the Sun the size of the Sun 1 Aristarchus, On the Sizes and Distances of the Sun and Moon

35 Other Early Scientific Measurements He came up with a way to measure the size of the Moon... 2 Total solar eclipse, Wikipedia, Luc Viatour. 3 Annular (ring) eclipse, Wikipedia, Je erson Teng.

36 Other Early Scientific Measurements He came up with a way to measure the size of the Moon... 2 Total solar eclipse, Wikipedia, Luc Viatour. 3 Annular (ring) eclipse, Wikipedia, Je erson Teng.

37 Other Early Scientific Measurements the size of the Moon the Moon s distance from the Earth the distance to the Sun the size of the Sun

38 Other Early Scientific Measurements the size of the Moon the Moon s distance from the Earth the distance to the Sun the size of the Sun diameter distance = 1 110

39 Other Early Scientific Measurements the size of the Moon the Moon s distance from the Earth the distance to the Sun the size of the Sun diam Moon = 3, 474 km diameter distance = dist Moon = 384, 400 km diam Sun = 1, 391, 684 km dist Sun = 149, 600, 000 km

40 What is Physics? We ve seen some examples early scientific measurements, but are they part of physics? Physics is the science of fundamental interactions of matter and energy.

41 What is Physics? We ve seen some examples early scientific measurements, but are they part of physics? Physics is the science of fundamental interactions of matter and energy. How is it done? Make a simplified model of the system of interest, then apply a principle to make a quantitative prediction.

42 What is Physics? We ve seen some examples early scientific measurements, but are they part of physics? Physics is the science of fundamental interactions of matter and energy. How is it done? Make a simplified model of the system of interest, then apply a principle to make a quantitative prediction. System Any physical object or group of objects about which we would like to make quantitative predictions.

43 What is Physics? System Any physical object or group of objects about which we would like to make quantitative predictions. eg. a pool table. The system might include the balls, the sides of the table, but maybe not the whole Earth. And certainly not the Andromeda galaxy. 1 Image from

44 What is Physics? Model A simplified description of a system and its interactions that includes only what is necessary to make predictions.

45 What is Physics? Model A simplified description of a system and its interactions that includes only what is necessary to make predictions. eg. the billiard balls are made up of atoms, but we can make very accurate predictions about their motion and collisions by just pretending they are uniform rigid spheres.

46 What is Physics? Model A simplified description of a system and its interactions that includes only what is necessary to make predictions. eg. the billiard balls are made up of atoms, but we can make very accurate predictions about their motion and collisions by just pretending they are uniform rigid spheres. Hypothesis An educated guess about a relationship between measurable quantities. It must be falsifiable by observations or experiments.

47 What is Physics? Theory A refined quantitative model for making predictions that has been verified by multiple groups of researchers and is understood to have some regime of validity.

48 What is Physics? Theory A refined quantitative model for making predictions that has been verified by multiple groups of researchers and is understood to have some regime of validity. eg. Newtonian Mechanics - very accurately predicts the motion of billiard balls and the motion of planets, but not the perihelion precession of Mercury, and not the behavior of electrons in atoms.

49 What is Physics? Theory A refined quantitative model for making predictions that has been verified by multiple groups of researchers and is understood to have some regime of validity. eg. Newtonian Mechanics - very accurately predicts the motion of billiard balls and the motion of planets, but not the perihelion precession of Mercury, and not the behavior of electrons in atoms. Valid when object s velocities v << c, the speed of light, gravitational fields are not too strong, distances are much bigger than `p (Planck length), etc.

50 What is Physics? Law A pattern in nature that can be described very concisely, often in a single mathematical equation.

51 What is Physics? Law A pattern in nature that can be described very concisely, often in a single mathematical equation. eg. F = m a ( If I push this shopping cart twice as hard, it will accelerate twice as fast. )

52 What is Physics? Theories and laws are expressed in terms of mathematics and equations. They are precise and quantitative. Mathematics is the language of science.

53 What Physical Theories Do You Know Of??

54 Quantities, Units, Measurement If we want to make quantitative statements we need to agree on measurements: standard reference units. We will mostly use SI (Système International) units: Length Mass Time meter, m kilogram, kg second, s and many more!

55 Quantities, Units, Measurement If we want to make quantitative statements we need to agree on measurements: standard reference units. We will mostly use SI (Système International) units: Length Mass Time meter, m kilogram, kg second, s and many more! Make sure you include units! Also, units can be helpful...

56 Defining Units Originally, units were arbitrary. One unit of length (the cubit) was the length of a forearm and hand. This is convenient, but not very precise since di erent people have di erent length arms.

57 Defining Units Originally, units were arbitrary. One unit of length (the cubit) was the length of a forearm and hand. This is convenient, but not very precise since di erent people have di erent length arms. Physicists now strive to chose definitions for units that are based on fundamental physical phenomena - things anyone, anywhere could in principle observe consistently.

58 Units: Length The meter, m, is the SI unit of length. It is about 3.28 feet. Originally (in 1793) the meter was defined so that the distance from the North Pole to the equator would be 10 million meters. However, this is not any easy distance to measure.

59 Units: Length The meter, m, is the SI unit of length. It is about 3.28 feet. Originally (in 1793) the meter was defined so that the distance from the North Pole to the equator would be 10 million meters. However, this is not any easy distance to measure. In 1799 a physical bar was made that was o have marks on it 1 meter apart. cially declared to

60 Units: Length The meter, m, is the SI unit of length. It is about 3.28 feet. Originally (in 1793) the meter was defined so that the distance from the North Pole to the equator would be 10 million meters. However, this is not any easy distance to measure. In 1799 a physical bar was made that was o have marks on it 1 meter apart. cially declared to The current definition of the meter (since 1983) is more precise and more convenient for experiments: meter One meter is the distance light travels in 1/299,792,458-ths of a second.

61 Units: Time The SI unit of time is the second, s. The second was originally defined (via the minute and hour) as 1/86,400-th of a day.

62 Units: Time The SI unit of time is the second, s. The second was originally defined (via the minute and hour) as 1/86,400-th of a day. All clocks are based on oscillating systems - systems that repeat the same motion over and over again.

63 Units: Time The SI unit of time is the second, s. The second was originally defined (via the minute and hour) as 1/86,400-th of a day. All clocks are based on oscillating systems - systems that repeat the same motion over and over again. For example, the Earth rotates over and over again. A pendulum swings back and forth.

64 Units: Time The SI unit of time is the second, s. The second was originally defined (via the minute and hour) as 1/86,400-th of a day. All clocks are based on oscillating systems - systems that repeat the same motion over and over again. For example, the Earth rotates over and over again. A pendulum swings back and forth. It is now more precisely defined in terms of the behavior of Cesium atoms. (Rubiduim and other elements are also used.)

65 Units: Time The second is now more precisely defined in terms of the behavior of Cesium atoms. (Rubiduim and other elements are also used.) Devices used to measure time this way are called atomic clocks. They are accurate to 1 second in 100 million years. 1 Atomic clock FOCS-1, METAS, Bern, Switzerland; Chip photo, NIST.

66 Units: Time Measuring time accurately is very important for navigation. Accurate clocks were needed to help ships determine their longitude (East-West position) in the 1700s. This lead to the development of clocks that worked based on the oscillations of springs rather than pendulums. 1 Harrison H5 naval chronometer. Photo from user Racklever, Wikipedia.

67 Units: Time Measuring time accurately is very important for navigation. Now most navigation systems use the Global Positioning System (GPS) a constellation of satellites carrying atomic clocks. 1 Image from GPS.gov.

68 Units: Mass The kilogram, kg, is the SI unit of mass. Loosely speaking, mass is a measure of the amount of matter in an object. The kilogram currently does not have a definition in terms of natural phenomena. 1 kilogram is 1,000 grams. Originally, the gram was defined to be the mass of one cubic centimeter of water at the melting point of water.

69 Units: Mass Now the o cial 1-kilogram sample, the international prototype kilogram is a cylinder of platinum and iridium stored near Paris. An alternative definition of the kilogram in terms of a fundamental constant has been proposed and at some point may be adopted. 1 Areplicaoftheprototypekilogram,Wikipedia.

70 Summary Scientific facts, hypotheses, theories, and laws Mearesurements Physics as modeling the natural world Homework Get the book: Conceptual Physics, 11th Edition, Hewitt Read Chapter 1